Found this on another forum: "Anytime the EVRC-B vocode suggestion is given, you should also mention that this does not work everywhere and your phone will not make telephone calls in areas that don't support this vocode (central North Carolina is one area). If you choose to do this remember that your phone might suddenly quit working while you are traveling. If so, change your vocode back to EVRC and try again before taking your phone to Verizon."
Could it be that you are in such an area?